When a transfer route of a sheet is curved in an image forming apparatus, the sheet may be curled when it passes through the curved route. The image forming apparatus of electronic photography is usually provided with a fixing unit for fixing a developer image on the sheet by heating and fusing it. When the sheet passes through the fixing unit, the moisture possibly evaporates from the sheet surface to curl the sheet. If the sheet is discharged while being curled, it may have a deteriorated stackability and may form an obscure image on the sheet.
It is general that the curl is corrected by clamping the sheet between rollers and a plurality of rolls thereby to curve the sheet along the circumferences of the rollers. However, the different sheets have different curling characteristics so that a constant uncurling force cannot possibly uncurl a highly tear-resistant sheet such as a postcard.
As disclosed in JP-A-7-179258, there is a technique, in which the force of a pair of rolls to push the transfer rollers are made so variable to adjust the uncurling force as to cover a plurality of kinds of sheets. By interposing springs between roll holders and movably supported spring seats and by displacing the spring seats by cams, more specifically, the force of the rolls to push the transfer rollers is changed.